1. Field of the Invention
The present invention relates to an injection drive mechanism for a servo injection molding machine. The injection drive mechanism includes an injection screw, a ball screw, and a spline shaft that are mounted along the same axis. The injection drive mechanism further includes two motors that may operate at various output speeds and/or operate in the same direction or opposite directions for providing various injection operations.
2. Description of the Related Art
A typical injection molding machine is actuated by hydraulic control that has problems of considerable energy consumption, oil leakage, and slow response to speed control. A servo-controlled injection molding machine may obviate the above problems, yet it is very complicated, as different injection speed control, pressure maintaining, material feeding, backpressure, etc are involved. An injection molding machine must be capable of controlling ordinary-speed injection or high-speed injection, and in some cases must provide low-speed/high-pressure injection. When the molten plastic material enters a mold cavity, the temperature of the plastic material begins to lower and the plastic material begins to solidify and thus shrink. At this time, pressure maintaining is required, and plastic material is supplied into the mold cavity to obtain a product with a precisely formed shape. The plastic material that moves forward in a barrel is stirred (by rotational movement of an injection screw) and heated by frictional heat resulting from shear force in the barrel, thereby performing the feeding/melting procedure. Meanwhile, when the molten plastic material is piled up in the barrel for subsequent injection, the injection screw must be moved backward very slowly to perform the backpressure control procedure, which affects the quality of the product to be formed.
U.S. Pat. No. 5,129,808 issued to Watanabe et al. on Jul. 14, 1992 discloses a two-plate type injection apparatus comprising a front plate, a pusher plate, and a single motor to drive two ball screws for actuating the pusher plate and a metering motor as well as corresponding feeding elements. Nevertheless, there are five axes involved, including two axes for two ball screws, two axes for two linear guides, and an axis for an injection screw. As a result, it is extremely difficult to keep the five axes parallel to each other. Making the ball screws, linear guides, and the injection screw move synchronously and controlling the precision are also difficult to achieve. In addition, such a servo-injection apparatus is heavy, consumes driving energy, and is incapable of performing high-speed injection.
U.S. Pat. No. 4,693,676 issued to Inaba on Sep. 15, 1987 discloses a screw-rotating/injection mechanism of an injection molding machine, wherein the front base and rear base are stationary and the pressure plate is movable. A servomotor drives two ball screws for actuating the pressure plate to thereby move a screw shaft. The other servomotor drives the screw shaft to rotate for feeding. Such an injection mechanism still has the drawbacks of heavy weight, consumption of driving energy, difficulty in keep several active axes parallel to each other, and difficulty in achieving synchronous control.
It is an object of the present invention to provide a servo injection molding machine to solve the above-mentioned drawbacks.
The servo injection molding machine in accordance with the present invention includes an injection screw, a ball screw, and a spline shaft that are mounted along the same axis. The spline shaft and the injection screw are connected by a connecting seat. An injection servomotor drives an injection sleeve mounted to a side of an injection seat, which, in turn, makes a ball nut drive a ball screw to move rectilinearly. A metering servomotor drives a spline shaft on a feeding seat to rotate in a direction that is same as or different from that of the injection servomotor or to rotate at a speed the same or not the same as that of the injection servomotor, thereby controlling the spline shaft to rotate on site or move rectilinearly for performing injection.
When the injection servomotor rotates in a direction, the metering servomotor does not rotate such that injection at normal speed is provided. When the metering servomotor rotates in a reverse direction, injection at high speed is provided by means reverse relative rotations between the ball screw and the ball nut. When the injection servomotor rotates in the direction and the metering servomotor rotates slowly in the reverse direction, low-speed/high-pressure injection is provided. The injection servomotor and the metering servomotor may rotate in the same direction to increase the injection speed or rotate in opposite directions to reduce the injection speed. Further, the injection servomotor and the metering servomotor may rotate at the same speed or different speeds to achieve control of various speeds.
After injection, speed of the injection servomotor is reduced. At last, supply of electricity continues and the injection servomotor does not rotate. This maintains torque without operation such that pressure maintaining is achieved in the mold cavity.
When the injection servomotor does not operate, operation of the metering servomotor is sufficient to melt the plastic material.
When the injection servomotor is controlled to rotate at low speed in the reverse direction, and the metering servomotor keeps on rotating for feeding, the injection screw is moved backward slowly to achieve formation of backpressure.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.